Organic light emitting diode display device

ABSTRACT

An organic light emitting diode display includes a driving transistor and a compensation transistor. The driving transistor includes a fist gate electrode disposed on a substrate, a polycrystalline semiconductor layer disposed on the first gate electrode of the driving transistor and including a first electrode, a second electrode, and a channel, and a second gate electrode disposed on the polycrystalline semiconductor layer of the driving transistor. The compensation transistor includes a polycrystalline semiconductor layer including a first electrode, a second electrode, and a channel, and a gate electrode disposed on the polycrystalline semiconductor layer of the compensation transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/356,142 filed Mar. 18, 2019, which claims priority under 35 U.S.C.§119 to Korean Patent Application No. 10-2018-0074950, filed in theKorean Intellectual Property Office on Jun. 28, 2018, the disclosures ofwhich are incorporated by reference herein in their entirety.

TECHNICAL FIELD

Exemplary embodiments of the present invention relate to an organiclight emitting diode display.

DISCUSSION OF THE RELATED ART

An organic light emitting diode display has a self-luminouscharacteristic. Since an organic light emitting diode display does notneed a separate light source, unlike a liquid crystal display, it canhave a relatively small thickness and weight. In addition, the organiclight emitting diode display exhibits high-quality characteristics suchas low power consumption, high luminance, high response speed, etc.

In general, an organic light emitting diode display includes asubstrate, a plurality of thin film transistors positioned on thesubstrate, a plurality of insulating layers disposed between wiresconfiguring the thin film transistors, and an organic light emittingelement connected to the thin film transistor.

SUMMARY

Exemplary embodiments of the present invention reduce a thickness of adisplay device and improve a display device by, for example, remove orreducing instantaneous afterimages despite the presence of aprotrusion(s) formed in a polycrystalline semiconductor of the displaydevice.

According to an exemplary embodiment, an organic light emitting diodedisplay includes a substrate, a pixel disposed on the substrate, a scanline, a data line, a driving voltage line, and an initialization voltageline. The scan line, the data line, the driving voltage line, and theinitialization voltage line are connected to the pixel. The pixelincludes an organic light emitting element, a first switching transistorconnected to the scan line, a driving transistor that applies a currentto the organic light emitting element, and a compensation transistorthat compensates an operation of the driving transistor. The drivingtransistor includes a first gate electrode disposed on the substrate, apolycrystalline semiconductor layer disposed on the first gate electrodeand including a first electrode, a second electrode, and a channel. Thecompensation transistor includes a polycrystalline semiconductor layerincluding a first electrode, a second electrode, and a channel, and afirst gate electrode disposed on the polycrystalline semiconductor layerof the compensation transistor.

In an exemplary embodiment, the driving transistor further includes asecond gate electrode disposed on the polycrystalline semiconductorlayer of the driving transistor.

In an exemplary embodiment, the second gate electrode of the drivingtransistor receives a driving voltage that flows to the driving voltageline.

In an exemplary embodiment, the driving transistor further includes anoverlapping layer disposed between the substrate and the first gateelectrode of the driving transistor.

In an exemplary embodiment, the driving voltage that flows to thedriving voltage line is applied to the overlapping layer.

In an exemplary embodiment, the organic light emitting diode displayfurther includes a driving voltage application part that applies thedriving voltage to the overlapping layer and the second gate electrodeof the driving transistor.

In an exemplary embodiment, the organic light emitting diode displayfurther includes a second switching transistor. The first switchingtransistor is connected to the scan line and the data line, and thesecond switching transistor is connected to the scan line and the firstgate electrode of the driving transistor.

In an exemplary embodiment, the first switching transistor includes apolycrystalline semiconductor layer including a first electrode, asecond electrode, and a channel, and a first gate electrode disposed onthe polycrystalline semiconductor layer of the first switchingtransistor.

In an exemplary embodiment, the first switching transistor furtherincludes a second gate electrode disposed under the polycrystallinesemiconductor layer of the first switching transistor. A driving voltagethat flows to the driving voltage line is applied to the second gateelectrode of the first switching transistor.

In an exemplary embodiment, the first switching transistor includes agate electrode disposed on the substrate, and a polycrystallinesemiconductor layer disposed on the gate electrode of the firstswitching transistor and including a first electrode, a secondelectrode, and a channel.

In an exemplary embodiment, the second switching transistor includes apolycrystalline semiconductor layer including a first electrode, asecond electrode, and a channel, and a first gate electrode disposed onthe polycrystalline semiconductor layer of the second switchingtransistor.

In an exemplary embodiment, the second switching transistor furtherincludes a second gate electrode disposed under the polycrystallinesemiconductor layer of the second switching transistor. The second gateelectrode of the second switching transistor receives a driving voltagethat flows to the driving voltage line.

In an exemplary embodiment, the first switching transistor includes apolycrystalline semiconductor layer including a first electrode, asecond electrode, and a channel, and a first gate electrode disposed onthe polycrystalline semiconductor layer of the first switchingtransistor.

In an exemplary embodiment, the first switching transistor furtherincludes a second gate electrode disposed under the polycrystallinesemiconductor layer of the first switching transistor. The second gateelectrode of the first switching transistor receives the driving voltagethat flows to the driving voltage line.

In an exemplary embodiment, the driving transistor further includes anoverlapping layer disposed between the substrate and the first gateelectrode of the driving transistor. The overlapping layer receives thedriving voltage, and the overlapping layer is electrically connected tothe second gate electrode of the second switching transistor such thatthe driving voltage is applied to the second gate electrode of thesecond switching transistor.

In an exemplary embodiment, the organic light emitting diode displayfurther includes a driving voltage application part. The drivingtransistor further includes a second gate electrode disposed on thepolycrystalline semiconductor layer of the driving transistor, and thedriving voltage application part applies the driving voltage to theoverlapping layer and the second gate electrode of the drivingtransistor.

In an exemplary embodiment, the second switching transistor includes agate electrode disposed on the substrate, and a polycrystallinesemiconductor layer disposed on the gate electrode of the secondswitching transistor and including a first electrode, a secondelectrode, and a channel.

In an exemplary embodiment, the compensation transistor initializes thefirst gate electrode of the driving transistor.

In an exemplary embodiment, the compensation transistor further includesa second gate electrode disposed under the polycrystalline semiconductorlayer of the compensation transistor. The second gate electrode of thecompensation transistor receives a driving voltage that flows to thedriving voltage line.

According to an exemplary embodiment, an organic light emitting diodedisplay includes a substrate, a pixel disposed on the substrate, a scanline, a data line, a driving voltage line, and an initialization voltageline. The scan line, the data line, the driving voltage line, and theinitialization voltage line are connected to the pixel. The pixelincludes an organic light emitting element, a first switching transistorconnected to the scan line, a driving transistor that applies a currentto the organic light emitting element, and a compensation transistorthat compensates an operation of the driving transistor. The drivingtransistor includes a first gate electrode disposed on the substrate, apolycrystalline semiconductor layer disposed on the first gate electrodeand including a first electrode, a second electrode, and a channel, anda second gate electrode disposed on the polycrystalline semiconductorlayer of the driving transistor. The compensation transistor includes apolycrystalline semiconductor layer including a first electrode, asecond electrode, and a channel, and a first gate electrode disposed onthe polycrystalline semiconductor layer of the compensation transistor.The compensation transistor does not include a second gate electrodedisposed under the polycrystalline semiconductor layer of thecompensation transistor.

According to exemplary embodiments of the present invention, although aprotrusion(s) may be formed in the polycrystalline semiconductor layer,since the gate electrode of the driving transistor is disposed under thepolycrystalline semiconductor layer, the thickness of the gateinsulating layer may be reduced and the thickness of the display devicemay be reduced. Also, because the gate electrode of the drivingtransistor is disposed under the polycrystalline semiconductor, if thethickness of the gate insulating layer is reduced, a characteristic(hysteresis) of the driving transistor is reduced such thatinstantaneous afterimages are not generated in the displayed image.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will become moreapparent by describing in detail exemplary embodiments thereof withreference to the accompanying drawings, in which:

FIG. 1 is an equivalent circuit diagram of one pixel of an organic lightemitting diode display according to an exemplary embodiment.

FIG. 2 is a timing diagram of signals applied to one pixel of an organiclight emitting diode display according to an exemplary embodiment.

FIG. 3 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

FIG. 4 is a view schematically showing a structure of overlying layersdue to a protrusion generated in a polycrystalline semiconductor.

FIG. 5 is an equivalent circuit diagram of one pixel of an organic lightemitting diode display according to an exemplary embodiment.

FIG. 6 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

FIG. 7 is an equivalent circuit diagram of one pixel of an organic lightemitting diode display according to an exemplary embodiment.

FIG. 8 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

FIG. 9 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

FIG. 10 is an equivalent circuit diagram of one pixel of an organiclight emitting diode display according to an exemplary embodiment.

FIG. 11 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

FIG. 12 is an equivalent circuit diagram of one pixel of an organiclight emitting diode display according to an exemplary embodiment.

FIG. 13 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

FIG. 14 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

FIG. 15 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention will be described morefully hereinafter with reference to the accompanying drawings. Likereference numerals may refer to like elements throughout theaccompanying drawings.

It will be understood that when an element such as a layer, film,region, or substrate is referred to as being “on” another element, itcan be directly on the other element or intervening elements may also bepresent.

Spatially relative terms, such as “beneath”, “below”, “lower”, “under”,“above”, “upper”, etc., may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” or“under” other elements or features would then be oriented “above” theother elements or features. Thus, the exemplary terms “below” and“under” can encompass both an orientation of above and below.

It will be understood that the terms “first,” “second,” “third,” etc.are used herein to distinguish one element from another, and theelements are not limited by these terms. Thus, a “first” element in anexemplary embodiment may be described as a “second” element in anotherexemplary embodiment.

Descriptions of features or aspects within each exemplary embodimentshould typically be considered as available for other similar featuresor aspects in other exemplary embodiments, unless the context clearlyindicates otherwise.

An organic light emitting diode display according to an exemplaryembodiment is described with reference to FIGS. 1 to 3.

FIG. 1 is an equivalent circuit diagram of one pixel of an organic lightemitting diode display according to an exemplary embodiment. FIG. 2 is atiming diagram of signals applied to one pixel of an organic lightemitting diode display according to an exemplary embodiment. FIG. 3 is across-sectional view of a plurality of transistors included in one pixelof an organic light emitting diode display according to an exemplaryembodiment (e.g., the exemplary embodiment illustrated in FIG. 1).

Referring to FIG. 1, a pixel PX of an organic light emitting diodedisplay includes a plurality of transistors T1, T2, T3, T4, T5, T6, T7,T8, and T9, a storage capacitor Cst, and an organic light emitting diodeOLED, which are connected to signal lines 127, 151, 152, 153, 158, 171,172, and 741.

Referring to FIG. 3, the driving transistor T1 includes a gate electrodeG1 (referred to as a driving gate electrode) disposed under apolycrystalline semiconductor layer. The polycrystalline semiconductorlayer includes S1, D1, and a channel disposed therebetween. In contrast,in the other transistors T2-T7, gate electrodes G2, G3, G4, G5, G6, andG7 are disposed on the polycrystalline semiconductor layer. In FIG.3,G4, G5, G6 and G7 are illustrated as G, S4, S5, S6 and S7 areillustrated as S, and D4, D5, D6 and D7 are illustrated as D. In thedriving transistor T1, the gate electrode G1 is not affected by theprotrusion of the polycrystalline semiconductor layer, unlike the gateelectrodes G2, G3, G4, G5, G6, and G7 of the other transistors T2-T7,which may be affected. In an exemplary embodiment, a main function ofsupplying the current to the organic light emitting diode OLED is doneusing the driving transistor T1, and the other transistors T2-T7 performonly a function of preparing or initializing the operation of thedriving transistor T1. As a result, the influence on the display qualitydue to the protrusion of the polycrystalline semiconductor layer may bereduced.

Referring to FIGS. 1 and 3, the driving transistor T1 providing thecurrent to the organic light emitting diode OLED further includes anoverlapping layer M1 and a second gate electrode G1-2 overlapping theoverlapping layer M1. For convenience of illustration, the overlappinglayer M1 is not shown in FIG. 1. According to an exemplary embodiment,the overlapping layer M1 may be omitted. The second gate electrode G1-2is disposed on a side opposite to the gate electrode G1 of the drivingtransistor T1, with the semiconductor layer in which the channel of thedriving transistor T1 is formed being disposed therebetween. Theoverlapping layer M1 is disposed under the gate electrode G1 of thedriving transistor T1. The second gate electrode G1-2 and theoverlapping layer M1 are electrically connected to each other, and adriving voltage ELVDD is applied thereto. A predetermined voltage (thedriving voltage ELVDD) is applied to the second gate electrode G1-2 suchthat the second gate electrode G1-2 does not function as the gateelectrode of the transistor. As shown in FIG. 3, the second gateelectrode G1-2 is disposed on the channel of the polycrystallinesemiconductor layer, which includes S1, D1, and a channel disposedtherebetween.

The pixel PX according to an exemplary embodiment includes a total ofseven transistors T1 to T7.

The seven transistors include the driving transistor T1 that providesthe current to the organic light emitting diode OLED, and also include asecond transistor T2 connected to a scan line 151 and a data line andthat provides the data voltage in the pixel PX. The third transistor T3is also connected to the scan line 151. The second transistor T2 and thethird transistor T3 connected to the scan line 151 may each be referredto as a switching transistor. The other transistors used for operatingthe organic light emitting diode OLED may include a fourth transistorT4, a fifth transistor T5, a sixth transistor T6, and a seventhtransistor T7, and each of these transistors may be referred to as acompensation transistor.

The plurality of signal lines 127, 151, 152, 153, 158, 171, 172, and 741may include the scan line 151, a previous scan line 152, a lightemission control line 153, a bypass control line 158, a data line 171, adriving voltage line 172, an initialization voltage line 127, and acommon voltage line 741. The bypass control line 158 may be a part ofthe previous scan line 152 or may be electrically connected thereto.

The scan line 151 is connected to a gate driver and transmits a scansignal Sn to the second transistor T2 and the third transistor T3. Theprevious scan line 152 is connected to the gate driver and transmits aprevious scan signal Sn−1 applied to the pixel PX disposed at theprevious stage to the fourth transistor T4. The light emission controlline 153 is connected to a light emission controller and transmits alight emission control signal EM controlling a time that the organiclight emitting diode OLED is emitted to the fifth transistor T5 and thesixth transistor T6. The bypass control line 158 transmits a bypasssignal GB to the seventh transistor T7, and may transmit the same signalas the previous scan signal Sn−1 according to an exemplary embodiment.

The data line 171 is a wire that transmits a data voltage Dm generatedfrom a data driver. A luminance at which the organic light emittingdiode OLED (also referred to as an organic light emitting device) isemitted is changed depending on the data voltage Dm. The driving voltageline 172 applies the driving voltage ELVDD, the initialization voltageline 127 transmits an initialization voltage Vint initializing thedriving transistor T1, and the common voltage line 741 applies a commonvoltage ELVSS. The voltages applied to the driving voltage line 172, theinitialization voltage line 127, and the common voltage line 741 may bea predetermined voltage, respectively.

Next, the plurality of transistors are described in detail.

The driving transistor T1 controls a magnitude of the current outputdepending on the applied data voltage Dm, and an output driving currentId is applied to the organic light emitting diode OLED. As a result, abrightness of the organic light emitting diode OLED is controlleddepending on the data voltage Dm. For this purpose, the first electrodeS1 (also referred to as an input terminal) of the driving transistor T1is disposed so as to receive the driving voltage ELVDD, and is connectedto the driving voltage line 172 via the fifth transistor T5. The firstelectrode S1 of the driving transistor T1 is also connected to thesecond electrode D2 of the second transistor T2, and thereby alsoreceives the data voltage Dm. The second electrode D1 (also referred toas an output terminal) is disposed so as to output the current towardthe organic light emitting diode OLED, and is connected to an anode ofthe organic light emitting diode OLED via the sixth transistor T6. Thegate electrode G1 is connected to one electrode (a second storageelectrode E2) of the storage capacitor Cst. Accordingly, the voltage ofthe gate electrode G1 is changed depending on the voltage stored to thestorage capacitor Cst, and as a result, the driving current Id output bythe driving transistor T1 is changed. The gate electrode G1 is disposedbetween the polycrystalline semiconductor layer and the substrate,thereby being disposed under the polycrystalline semiconductor layer.Also, the second gate electrode G1-2 is disposed on the polycrystallinesemiconductor layer and directly receives the driving voltage ELVDD. Thedriving voltage ELVDD is also applied to the overlapping layer M1electrically connected to the second gate electrode G1-2, and theoverlapping layer M1 is disposed between the gate electrode G1 and thesubstrate. The second gate electrode G1-2 shifts a characteristic (athreshold voltage value) of the driving transistor T1 due to the drivingvoltage ELVDD in the channel region (a region between D1 and S1) of thepolycrystalline semiconductor such that the characteristic of thedriving transistor T1 is improved.

The second transistor T2 accepts the data voltage Dm in the pixel PX.The gate electrode G2 is connected to the scan line 151, and the firstelectrode S2 is connected to the data line 171. The second electrode D2of the second transistor T2 is connected to the first electrode S1 ofthe driving transistor T1. When the second transistor T2 is turned onaccording to the scan signal Sn transmitted through the scan line 151,the data voltage Dm transmitted through the data line 171 is transmittedto the first electrode S1 of the driving transistor T1. The gateelectrode G2 is disposed on the polycrystalline semiconductor layer inwhich the channel of the second transistor T2 is disposed.

The third transistor T3 transmits the compensation voltage (the voltageDm+Vth) of which the data voltage Dm is changed by the drivingtransistor T1 to the second storage electrode E2 of the storagecapacitor Cst. The gate electrode G3 of the third transistor T3 isconnected to the scan line 151. The first electrode S3 of the thirdtransistor T3 is connected to the second electrode D1 of the drivingtransistor T1, and the second electrode D3 of the third transistor T3 isconnected to the second storage electrode E2 of the storage capacitorCst and the gate electrode G1 of the driving transistor T1. The thirdtransistor T3 is turned on depending on the scan signal Sn transmittedthrough the scan line 151. When turned on, the third transistor T3connects the gate electrode G1 and the second electrode D1 of thedriving transistor T1, and connects the second electrode D1 of thedriving transistor T1 and the second storage electrode E2 of the storagecapacitor Cst. The gate electrode G3 is disposed on the polycrystallinesemiconductor layer in which the channel of the third transistor T3 isdisposed.

The fourth transistor T4 has a function of initializing the gateelectrode G1 of the driving transistor T1 and the second storageelectrode E2 of the storage capacitor Cst. The gate electrode G4 of thefourth transistor T4 is connected to the previous scan line 152 and thefirst electrode S4 is connected to the initialization voltage line 127.The second electrode D4 of the fourth transistor T4 is connected to thesecond storage electrode E2 of the storage capacitor Cst and the gateelectrode G1 of the driving transistor T1 via the second electrode D3 ofthe third transistor T3. The fourth transistor T4 transmits theinitialization voltage Vint to the gate electrode G1 of the drivingtransistor T1 and the second storage electrode E2 of the storagecapacitor Cst depending on the previous scan signal Sn−1 transmittedthrough the previous scan line 152. Accordingly, the gate voltage of thegate electrode G1 of the driving transistor T1 and the storage capacitorCst are initialized. The initialization voltage Vint may be a voltagethat is capable of turning on the driving transistor T1 by having a lowvoltage value. The gate electrode G4 is disposed on the polycrystallinesemiconductor layer in which the channel of the fourth transistor T4 isdisposed.

The fifth transistor T5 has a function of transmitting the drivingvoltage ELVDD to the driving transistor T1. The gate electrode G5 isconnected to the light emission control line 153 and the first electrodeS5 is connected to the driving voltage line 172. The second electrode D5of the fifth transistor T5 is connected to the first electrode S1 of thedriving transistor T1. The gate electrode G5 is disposed on thepolycrystalline semiconductor layer in which the channel of the fifthtransistor T5 is disposed.

The sixth transistor T6 has a function of transmitting the drivingcurrent Id output from the driving transistor T1 to the organic lightemitting diode OLED. The gate electrode G6 is connected to the lightemission control line 153, and the first electrode S6 is connected tothe second electrode D1 of the driving transistor T1. The secondelectrode D6 of the sixth transistor T6 is connected to the anode of theorganic light emitting diode OLED. The gate electrode G6 is disposed onthe polycrystalline semiconductor layer in which the channel of thesixth transistor T6 is disposed.

The fifth transistor T5 and the sixth transistor T6 are simultaneouslyturned on depending on the light emission control signal EM transmittedthrough the light emission control line 153. When the driving voltageELVDD is transmitted to the first electrode S1 of the driving transistorT1 through the fifth transistor T5, the driving transistor T1 outputsthe driving current Id depending on the voltage (e.g., the voltage ofthe second storage electrode E2 of the storage capacitor Cst) of thegate electrode G1 of the driving transistor T1. The output drivingcurrent Id is transmitted to the organic light emitting diode OLEDthrough the sixth transistor T6. While a current Ioled flows to theorganic light emitting diode OLED, the organic light emitting diode OLEDemits the light.

The seventh transistor T7 has a function of initializing the anode ofthe organic light emitting diode OLED. The gate electrode G7 isconnected to the bypass control line 158, the first electrode S7 isconnected to the anode of the organic light emitting diode OLED, and thesecond electrode D7 is connected to the initialization voltage line 127.The bypass control line 158 may be connected to the previous scan line152, and the bypass signal GB is applied with the signal with the sametiming as the previous scan signal Sn−1. In an exemplary embodiment, thebypass control line 158 is not connected to the previous scan line 152,and may transmit a separate signal from the previous scan signal Sn−1.When the seventh transistor T7 is turned on depending on the bypasssignal GB, the initialization voltage Vint is applied to the anode ofthe organic light emitting diode OLED to be initialized. The gateelectrode G7 is disposed on the polycrystalline semiconductor layer inwhich the channel of the seventh transistor T7 is disposed.

The pixel PX also includes the storage capacitor Cst. The data voltagepasses through the driving transistor T1 and is applied to the storagecapacitor Cst to be stored.

A first storage electrode E1 of the storage capacitor Cst is connectedto the driving voltage line 172, and the second storage electrode E2 isconnected to the gate electrode G1 of the driving transistor T1, thesecond electrode D3 of the third transistor T3, and the second electrodeD4 of the fourth transistor T4. As a result, the second storageelectrode E2 determines the voltage (the gate-source voltage Vgs of thedriving transistor T1) of the gate electrode G1 of the drivingtransistor T1, the data voltage Dm is applied through the secondelectrode D3 of the third transistor T3, and the initialization voltageVint is applied through the second electrode D4 of the fourth transistorT4.

The pixel PX also includes the organic light emitting diode OLED, theanode of the organic light emitting diode OLED is connected to thesecond electrode D6 of the sixth transistor T6 and the first electrodeS7 of the seventh transistor T7, and the cathode is connected to thecommon voltage line 741 that transmits the common voltage ELVSS.

In the exemplary embodiment of FIG. 1, the pixel circuit includes seventransistors T1-T7 and one capacitor Cst. However, the pixel circuit isnot limited thereto. For example, according to exemplary embodiments,the number of transistors, the number of capacitors, and theirconnections may be variously changed.

The organic light emitting diode display includes a display area inwhich an image is displayed, and the pixels PX are arranged in variousforms such as a matrix in the display area.

The operation of one pixel of the organic light emitting diode displayaccording to an exemplary embodiment is now described with reference toFIGS. 1 and 2.

During an initialization period, the previous scan signal Sn−1 of a lowlevel is supplied to the pixel PX through the previous scan line 152.Thus, the fourth transistor T4 that receives previous scan signal Sn−1is turned on, and the initialization voltage Vint is applied to the gateelectrode G1 of the driving transistor T1 and the second storageelectrode E2 of the storage capacitor Cst through the fourth transistorT4. As a result, the driving transistor T1 and the storage capacitor Cstare initialized. The initialization voltage Vint is a low voltage suchthat the driving transistor T1 may be turned on.

During the initialization period, the bypass signal GB of a low level isalso applied to the seventh transistor T7. Thus, the seventh transistorT7 that receives the bypass signal GB is turned on such that theinitialization voltage Vint is applied to the anode of the organic lightemitting diode OLED through the seventh transistor T7. As a result, theanode of the organic light emitting diode OLED is also initialized.

Next, during a data writing period (hereinafter referred to as a writingperiod), the scan signal Sn of a low level is supplied to the pixel PXthrough the scan line 151. The second transistor T2 and the thirdtransistor T3 are turned on by the scan signal Sn of a low level.

When the second transistor T2 is turned on, the data voltage Dm is inputto the first electrode S1 of the driving transistor T1 after passingthrough the second transistor T2.

Also, during the data writing period, the third transistor T3 is turnedon, and as a result, the second electrode D2 of the driving transistorT1 is electrically connected to the gate electrode G1 and the secondstorage electrode E2 of the storage capacitor Cst. The gate electrode G1and the second electrode D2 of the driving transistor T1 are connectedto be diode-connected. Also, the low voltage (the initialization voltageVint) is applied to the gate electrode G1 of the driving transistor T1during the initialization period such that the driving transistor T1 isin the turned-on state. As a result, the data voltage Dm input to thefirst electrode S1 of the driving transistor T1 passes through thechannel of the driving transistor T1 and is output from the secondelectrode D1, and is then stored in the second storage electrode E2 ofthe storage capacitor Cst through the third transistor T3. In this case,the voltage applied to the second storage electrode E2 is changedaccording to the threshold voltage Vth of the driving transistor T1, andwhen the data voltage Dm is applied to the first electrode S1 of thedriving transistor T1 and the initialization voltage Vint is applied tothe gate electrode G1 of the driving transistor T1, the voltage outputto the second electrode D1 may have the value (Vgs+Vth). Here, thevoltage Vgs is a difference between the voltages applied to the gateelectrode G1 and the first electrode S1 of the driving transistor T1,thereby having the value (Dm−Vint). Therefore, the voltage output fromthe second electrode D1 and stored in the second storage electrode E2may have the value (Dm−Vint+Vth).

Next, during the light emission period, the light emission controlsignal EM supplied from the light emission control line 153 has thevalue of a low level such that the fifth transistor T5 and the sixthtransistor T6 are turned on.

Since the fifth transistor T5 and the sixth transistor T6 are turned on,the driving voltage ELVDD is applied to the first electrode S1 of thedriving transistor T1, and the second electrode D1 of the drivingtransistor T1 is connected to the organic light emitting diode OLED. Thedriving current Id is generated depending on the voltage differencebetween the voltage of the gate electrode G1 of the driving transistorT1 and the voltage (e.g., the driving voltage ELVDD) of the firstelectrode S1. The driving current Id of the driving transistor T1 mayhave a value that is proportional to the square of the value (Vgs−Vth).Here, the value Vgs is the same as the difference in voltages applied toboth terminals of the storage capacitor Cst, and the value Vgs is(Vg−Vs), thereby having the value (Dm−Vint+Vth−ELVDD). Here, when thevalue (Vgs−Vth) is obtained by subtracting the value Vth, the value(Dm−Vint−ELVDD) is obtained. That is, the driving current Id of thedriving transistor T1 has the value regardless of the threshold voltageVth of the driving transistor T1.

Therefore, although the driving transistors T1 disposed in each pixel PXhave the different threshold voltages Vth due to a process distribution,the output current of each driving transistor T1 may be constant,thereby improving a non-uniformity characteristic thereof.

In the driving transistor T1, the gate electrode G1 is formed under thepolycrystalline semiconductor layer. As a result, even if the protrusionis formed in the polycrystalline semiconductor layer, the drivingtransistor T1 is operated properly regardless of the protrusion (e.g.,the protrusion does not negatively affect the driving transistor T1),and a constant characteristic is obtained. As a result, the displaydevice may be free from display defects such as instantaneousafterimages.

In the above equation, the value Vth may have a value that is slightlylarger than 0 or a negative value in the case of the P-type transistorusing the polycrystalline semiconductor. Also, expression of + and − maybe changed according to a direction in which the voltage is calculated.However, there is no change in the point that the driving current Id asthe output current of the driving transistor T1 may have a value thatdoes not depend on the threshold voltage Vth.

When the above-described light emission period is ended, theinitialization period is again started and the same operations arerepeated from the beginning.

For the first electrode and the second electrode of the plurality oftransistors T1, T2, T3, T4, T5, T6, and T7, one may be the sourceelectrode S and the other may be the drain electrode D according to thedirection of the application of the voltage or the current.

According to an exemplary embodiment, while the seventh transistor T7 inthe initialization period initializes the anode of the organic lightemitting diode OLED, a small amount of the current emitted in thecondition that the driving transistor T1 is turned on may also beprevented from flowing toward the organic light emitting diode OLED. Inthis case, the small amount of the current is discharged as a bypasscurrent Ibp to the initialization voltage Vint terminal through theseventh transistor T7. As a result, since the organic light emittingdiode OLED does not emit unnecessary light, a black gray may be moreclearly displayed and a contrast ratio may also be improved. In thiscase, the bypass signal GB may be a signal of different timing from thatof the previous scan signal Sn−1. According to an exemplary embodiment,the seventh transistor T7 may be omitted.

Further, in the above-operated pixel PX, as the driving voltage ELVDD isapplied to the second gate electrode G1-2 of the driving transistor T1,the characteristic (the threshold voltage) of the driving transistor T1is shifted such that the display quality is improved.

Next, a cross-sectional structure of transistors included in the organiclight emitting diode display is described with reference to FIG. 3.

FIG. 3 shows the cross-section of a plurality of transistors included inone pixel of the organic light emitting diode display. From left toright, the driving transistor T1, the third transistor T3, and thesecond transistor T2 are shown, followed by the fourth transistor T4 tothe seventh transistor T7. The cross-sections of the fourth transistorT4 to the seventh transistor T7 are the same. Thus, for convenience ofillustration, these cross-sections are grouped together and shown asone. In this regard, in FIG. 3, G corresponds to G4 to G7, D correspondsto D4 to D7, and S corresponds to S4 to S7.

According to an exemplary embodiment, the organic light emitting diodedisplay includes substrates 110 and 110-1 including, for example,plastic or polyimide (PI), and barrier layers 111 and 111-1 respectivelydisposed thereon. The substrates 110 and 110-1 may be collectivelyreferred to as a substrate, and the barrier layers 111 and 111-1 may becollectively referred to as a barrier layer. According to exemplaryembodiments, the substrate and the barrier layer may be formed with thesame number, and unlike as shown in FIG. 3, they may contain only onepair, or three or more pairs may be formed. The barrier layers 111 and111-1 may be formed to reduce an influence applied to the flexiblesubstrates 110 and 110-1 when forming overlying layers.

Now, the cross-section of the driving transistor T1 is described.

The overlapping layer M1 is disposed on the upper barrier layer 111-1,and the overlapping layer is covered by a buffer layer 112. The gateelectrode G1 is formed on the buffer layer 112, and the gate electrodeG1 is covered by a first gate insulating layer 141. The polycrystallinesemiconductor layer is disposed on the first gate insulating layer 141.The polycrystalline semiconductor layer includes the first electrode S1,the second electrode D1, and the channel layer disposed therebetween.

The polycrystalline semiconductor layer is covered by a second gateinsulating layer 142, and the second gate electrode G1-2 is formed onthe second gate insulating layer 142. The second gate electrode G1-2 iscovered by an interlayer insulating layer 160.

A data conductor is formed on the interlayer insulating layer 160. Thedata conductor includes the data line 171 and the driving voltage line172 that transmits the driving voltage ELVDD.

The driving voltage line 172 includes a driving voltage application partC-1 connected to the second gate electrode G1-2 and the overlappinglayer M1 through openings respectively exposing the second gateelectrode G1-2 and the overlapping layer M1. The driving voltageapplication part C-1 may be a part from which the driving voltage line172 extends or a part that is only electrically connected.

The gate electrode G1 and the second gate electrode G1-2 are disposedabove and below the channel of the driving transistor T1. In anexemplary embodiment, the gate electrode G1 and the second gateelectrode G1-2 may have a width corresponding to (e.g., substantiallyequal to) the width of the channel. However, in an exemplary embodiment,as shown in FIG. 3, the structure of the gate electrode G1 extendstoward the third transistor T3 (e.g., toward the right in FIG. 3) suchthat the gate electrode G1 disposed under the channel is connected tothe second electrode D3 of the third transistor T3. In an exemplaryembodiment, except for this extension part, the gate electrode G1 hasthe width corresponding to (e.g., substantially equal to) the width ofthe channel of the driving transistor T1.

The second gate electrode G1-2 may be used as a mask when doping thepolycrystalline semiconductor layer. As a result, the width of thesecond gate electrode G1-2 may accord with the width of the channel.According to an exemplary embodiment, as the driving transistor T1includes the second gate electrode G1-2 and the overlapping layer M1 asa part receiving the driving voltage ELVDD, the overlapping layer M1 maybe omitted.

The driving transistor T1 has a bottom gate (e.g., the second gateelectrode G1-2), and the driving voltage ELVDD is applied to the secondgate electrode G1-2, thereby shifting the characteristic of the channel.As a result, according to exemplary embodiments, defects that may begenerated due to a protrusion(s) (see FIG. 4) formed in thepolycrystalline semiconductor may be reduced or eliminated by using thebottom gate.

Next, the cross-section of the third transistor T3 is described.

The buffer layer 112 is disposed on the upper barrier layer 111-1, andthe first gate insulating layer 141 is disposed on the buffer layer 112.

The polycrystalline semiconductor layer is disposed on the first gateinsulating layer 141. The polycrystalline semiconductor layer includesthe first electrode S3, the second electrode D3, and the channel layerdisposed therebetween.

The polycrystalline semiconductor layer is covered by the second gateinsulating layer 142, and the gate electrode G3 (151) is formed on thesecond gate insulating layer 142. The gate electrode G3 (151) is coveredby the interlayer insulating layer 160.

A connection part 71 is formed on the interlayer insulating layer 160.The connection part 71 electrically connects the second electrode D3 ofthe third transistor T3 and the gate electrode G1 of the drivingtransistor T1 through openings respectively exposing the secondelectrode D3 of the third transistor T3 and the gate electrode G1 of thedriving transistor T1. Since the output of the driving transistor T1 ischanged depending the voltage of the gate electrode G1, the voltageoutput to the second electrode D3 of the third transistor T3 affects theoutput of the driving transistor T1.

The gate electrode G3 is disposed on the channel of the third transistorT3, and has a width corresponding to (e.g., substantially equal to) thewidth of the channel. The gate electrode G3 may be used as a mask whendoping the polycrystalline semiconductor layer.

As described above, unlike the driving transistor T1, in an exemplaryembodiment, the third transistor T3 has the top gate and does notinclude a bottom gate. Thus, unlike the driving transistor T1, in anexemplary embodiment, the third transistor T3 does not include astructure that shifts the characteristic of the channel.

Next, the cross-section of the second transistor T2 is described.

The buffer layer 112 is disposed on the upper barrier layer 111-1, andthe first gate insulating layer 141 is disposed on the buffer layer 112.

The polycrystalline semiconductor layer is disposed on the first gateinsulating layer 141. The polycrystalline semiconductor layer includesthe first electrode S2, the second electrode D2, and the channel layerdisposed therebetween.

The polycrystalline semiconductor layer is covered by the second gateinsulating layer 142, and the gate electrode G2 (151) is formed on thesecond gate insulating layer 142. The gate electrode G2 (151) is coveredby the interlayer insulating layer 160.

The data line 171 is formed on the interlayer insulating layer 160. Thedata line 171 is connected to the first electrode S2 of the secondtransistor T2 through an opening exposing the first electrode S2 of thesecond transistor T2. When the second transistor T2 is turned on, thedata voltage is input to the corresponding pixel PX.

The gate electrode G2 is disposed on the channel of the secondtransistor T2, and has a width corresponding to (e.g., substantiallyequal to) the width of the channel. The gate electrode G2 may be used asa mask when doping the polycrystalline semiconductor layer.

As described above, in an exemplary embodiment, unlike the drivingtransistor T1, the second transistor T2 has the top gate and does notinclude a bottom gate. Thus, in an exemplary embodiment, the secondtransistor T2 does not include a structure that shifts thecharacteristic of the channel.

Hereinafter, the fourth to seventh transistors T4-T7 included in thepixel PX are grouped and described. The first electrode of eachtransistor is indicated by S, the second electrode thereof is indicatedby D, and the gate electrode thereof is indicated by G.

The buffer layer 112 is disposed on the upper barrier layer 111-1, andthe first gate insulating layer 141 is disposed on the buffer layer 112.The polycrystalline semiconductor layer is disposed on the first gateinsulating layer 141. The polycrystalline semiconductor layer includesthe first electrode S, the second electrode D, and the channel layerdisposed therebetween.

The polycrystalline semiconductor layer is covered by the second gateinsulating layer 142, and the gate electrode G is formed on the secondgate insulating layer 142. The gate electrode G is covered by theinterlayer insulating layer 160.

The fourth to seventh transistors T4 to T7 have the connectionrelationship shown in FIG. 1 and are disposed in the pixel PX.

The gate electrode G is disposed on the channel of the fourth to seventhtransistors T4 to T7, and has a width corresponding to (e.g.substantially equal to) the width of the channel. The gate electrode Gmay be used as a mask when doping the polycrystalline semiconductorlayer.

As described above, in an exemplary embodiment, the fourth to seventhtransistors T4 to T7 also have the top gate and do not include a bottomgate. As a result, in an exemplary embodiment, the fourth to seventhtransistors T4 to T7 do not include a structure that shifts thecharacteristic of the channel.

In the organic light emitting diode display having the above-describedstructure, only the driving transistor T1 executing the main operationin the pixel PX has the bottom gate (the gate electrode disposed underthe polycrystalline semiconductor layer), and the rest of thetransistors have the top gate (the gate electrode disposed on thepolycrystalline semiconductor layer) and do not have a bottom gate.

The polycrystalline semiconductor layer is formed by forming thesemiconductor layer of amorphous silicon and irradiating a laser to thesemiconductor layer for crystallization. A protrusion(s) may be formedin the polycrystalline semiconductor layer in the crystallization step,and the expanded cross-section in the structure using the top gate isdescribed with reference to FIG. 4.

FIG. 4 is a view schematically showing a structure of overlying layersdue to a protrusion generated in a polycrystalline semiconductor.

The cross-sectional view in FIG. 4 shows the structure using the topgate in the driving transistor T1. Two gate layers GL1 and GL2 areformed on the polycrystalline semiconductor layer Poly.

The polycrystalline semiconductor layer Poly having the protrusion isdisposed on the substrate 110. The protrusion formed in thepolycrystalline semiconductor layer Poly is formed so as to form theconvex protrusion in the first and second gate insulating layers 141 and142 and the two gate layers GL1 and GL2 disposed thereon. As a result,the charges are gathered on the protrusion of the gate layers GL1 andGL2, and the thickness of the first and second gate insulating layers141 and 142 becomes thin such that the insulation aspect may bedestroyed. To prevent the insulation breakdown, the thickness of thefirst and second gate insulating layers 141 and 142 should besufficiently thick, and as a result, there is a limit in reduction ofthe thickness of the display device.

However, in exemplary embodiments according to the present invention,the gate electrode G1 of the driving transistor T1 is formed under theprotrusion of the polycrystalline semiconductor (the bottom gate), andas a result, the influence of the protrusion is reduced or eliminated.That is, even if the thickness of the gate insulating layer 141 is madethin, sufficient insulating characteristics may be obtained.

Unlike the driving transistor T1, the second to seventh transistorsT2-T7 have the top gate structure (e.g., they do not include a bottomgate) and the gate electrode is disposed on the polycrystallinesemiconductor layer including the protrusion. However, the second toseventh transistors T2-T7 do not play a major role in providing thecurrent to the organic light emitting diode OLED, and thus, the displayquality is not particularly influenced as a result of protrusionscorresponding to the second to seventh transistors T2-T7. However, in anexemplary embodiment, at least some transistors from among the second toseventh transistors T2-T7 may have the bottom gate structure to furtherimprove display characteristics.

In the exemplary embodiment of FIGS. 1 to 3, the characteristic of thedriving transistor T1 is shifted by forming the second gate electrodeG1-2 that applies the driving voltage ELVDD in only the drivingtransistor T1. However, exemplary embodiments of the present inventionare not limited thereto. For example, according to an exemplaryembodiment, at least some transistors from among the second to seventhtransistors T2-T7 may include the second gate electrode that applies thedriving voltage ELVDD. Utilizing the second gate electrode in some ofthe second to seventh transistors T2-T7 may increase the complexity andcost of a display device compared to utilizing the second gate electrodeG1-2 in only the driving transistor T1, but may further improve displaycharacteristics.

Next, an exemplary embodiment further including a second gate electrodeG2-2 that applies the driving voltage to the second transistor T2 isdescribed with reference to FIGS. 5 and 6.

FIG. 5 is an equivalent circuit diagram of one pixel of an organic lightemitting diode display according to an exemplary embodiment. FIG. 6 is across-sectional view of a plurality of transistors included in one pixelof an organic light emitting diode display according to an exemplaryembodiment (e.g., the exemplary embodiment illustrated in FIG. 5).

Compared to the circuit diagram of FIG. 1, in the circuit diagram ofFIG. 5, the second transistor T2 further includes the second gateelectrode G2-2, and the second gate electrode G2-2 is connected to thedriving voltage line 172. Thus, the second gate electrode G2-2 receivesthe driving voltage ELVDD, and as a result, the channel characteristicof the second transistor T2 is shifted in addition to the channelcharacteristic of the driving transistor T1.

Compared to the cross-sectional view of FIG. 3, in the cross-sectionalview of FIG. 6, the second transistor T2 further incudes the second gateelectrode G2-2, and a driving voltage application part C-2 that appliesthe driving voltage ELVDD to the second gate electrode G2-2 is furtherincluded.

Referring to FIGS. 5 and 6, for convenience of explanation, a furtherdescription of elements previously described may be omitted.

The cross-section of the second transistor T2 is now described indetail.

The buffer layer 112 is disposed on the upper barrier layer 111-1, andthe second gate electrode G2-2 is disposed on the buffer layer 112. Thesecond gate electrode G2-2 is covered by the first gate insulating layer141.

The polycrystalline semiconductor layer is disposed on the first gateinsulating layer 141. The polycrystalline semiconductor layer includesthe first electrode S2, the second electrode D2, and the channel layerdisposed therebetween.

The polycrystalline semiconductor layer is covered by the second gateinsulating layer 142, and the gate electrode G2 (151) is formed on thesecond gate insulating layer 142. The gate electrode G2 (151) is coveredby the interlayer insulating layer 160.

The data line 171 and the driving voltage application part C-2 areformed on the interlayer insulating layer 160. The data line 171 isconnected to the first electrode S2 of the second transistor T2 throughan opening exposing the first electrode S2 of the second transistor T2.When the second transistor T2 is turned on, the data voltage is input tothe corresponding pixel PX. The driving voltage application part C-2 isconnected to the second gate electrode G2-2 of the second transistor T2through an opening exposing the second gate electrode G2-2. The drivingvoltage application part C-2 applies the driving voltage ELVDD to thesecond gate electrode G2-2. As a result, the channel characteristic ofthe second transistor T2 is shifted.

The gate electrode G2 is disposed on the channel of the secondtransistor T2, and has a width corresponding to (e.g., substantiallyequal to) the width of the channel. The gate electrode G2 may be used asa mask when doping the polycrystalline semiconductor layer.

As described above, the second transistor T2 includes the top gate. Inaddition, in the exemplary embodiment of FIGS. 5 and 6, the second gateelectrode G2-2 that applies the driving voltage ELVDD is additionallyincluded such that the channel characteristic of the second transistorT2 is also shifted.

According to an exemplary embodiment, the second transistor T2 may beformed with the bottom gate, and the second gate electrode may be formedwith the top gate. In this case, the gate electrode G2 may be formedunder the polycrystalline semiconductor layer such that it is connectedto the scan line 151, and the second gate electrode G2-2 may be formedon the polycrystalline semiconductor layer so that the driving voltageELVDD is applied.

Next, an exemplary embodiment further including a second gate electrodeG3-2 that applies the driving voltage to the third transistor T3 isdescribed with reference to FIGS. 7 and 8.

FIG. 7 is an equivalent circuit diagram of one pixel of an organic lightemitting diode display according to an exemplary embodiment. FIG. 8 is across-sectional view of a plurality of transistors included in one pixelof an organic light emitting diode display according to an exemplaryembodiment (e.g., the exemplary embodiment illustrated in FIG. 7).

Compared to the circuit diagram of FIG. 1, in the circuit diagram ofFIG. 7, the third transistor T3 further includes the second gateelectrode G3-2, and the second gate electrode G3-2 is connected to thedriving voltage line 172. Thus, the second gate electrode G3-2 receivesthe driving voltage ELVDD, and as a result, the channel characteristicof the third transistor T3 is shifted in addition to the channelcharacteristic of the driving transistor T1.

Compared to the cross-sectional view of FIG. 1, in the cross-sectionalview of FIG. 8, the third transistor T3 further includes the second gateelectrode G3-2, and the driving voltage application part C-3 thatapplies the driving voltage ELVDD to the second gate electrode G3-2 isfurther included.

Referring to FIGS. 7 and 8, for convenience of explanation, a furtherdescription of elements previously described may be omitted.

The cross-section of the third transistor T3 is now described in detail.

The buffer layer 112 is disposed on the upper barrier layer 111-1, andthe second gate electrode G3-2 is disposed on the buffer layer 112. Thesecond gate electrode G3-2 is covered by the first gate insulating layer141.

The polycrystalline semiconductor layer is disposed on the first gateinsulating layer 141. The polycrystalline semiconductor layer includesthe first electrode S3, the second electrode D3, and the channel layerdisposed therebetween.

The polycrystalline semiconductor layer is covered by the second gateinsulating layer 142, and the gate electrode G3 (151) is formed on thesecond gate insulating layer 142. The gate electrode G3 (151) is coveredby the interlayer insulating layer 160.

The connection part 71 and the driving voltage application part C-3 areformed on the interlayer insulating layer 160.

The connection part 71 electrically connects the second electrode D3 ofthe third transistor T3 and the gate electrode G1 of the drivingtransistor T1 through openings respectively exposing the secondelectrode D3 of the third transistor T3 and the gate electrode G1 of thedriving transistor T1. Since the output of the driving transistor T1 ischanged according to the voltage of the gate electrode G1, the voltageoutput to the second electrode D3 of the third transistor T3 affects theoutput of the driving transistor T1.

The driving voltage application part C-3 is connected to the second gateelectrode G3-2 of the third transistor T3 through an opening exposingthe second gate electrode G3-2. The driving voltage application part C-3applies the driving voltage ELVDD to the second gate electrode G3-2. Asa result, the channel characteristic of the third transistor T3 isshifted.

The gate electrode G3 is disposed on the channel of the third transistorT3, and has a width corresponding to (e.g., substantially equal to) thewidth of the channel. The gate electrode G3 may be used as a mask whendoping the polycrystalline semiconductor layer.

As described above, the third transistor T3 includes the top gate. Inaddition, in the exemplary embodiment of FIGS. 7 and 8, the second gateelectrode G3-2 that applies the driving voltage ELVDD is additionallyincluded such that the channel characteristic of the third transistor T3is also shifted.

According to an exemplary embodiment, the third transistor T3 may beformed with the bottom gate, and the second gate electrode may be formedwith the top gate. In this case, the gate electrode G3 is formed underthe polycrystalline semiconductor layer such that it is connected to thescan line 151, and the second gate electrode G3-2 is formed on thepolycrystalline semiconductor layer so as to apply the driving voltageELVDD.

Next, a method for applying the driving voltage ELVDD to the second gateelectrode G3-2 of the third transistor T3 is described with reference toFIG. 9.

FIG. 9 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

The cross-sectional structure of each of the transistors T1 to T7 ofFIG. 9 is the same as that of FIG. 8. However, unlike the exemplaryembodiment illustrated in FIG. 8, in the exemplary embodimentillustrated in FIG. 9, the driving voltage application part C-3 thatapplies the driving voltage ELVDD to the second gate electrode G3-2 ofthe third transistor T3 is omitted.

Instead, in the exemplary embodiment illustrated in FIG. 9, the secondgate electrode G3-2 of the third transistor T3 receives the drivingvoltage ELVDD from the overlapping layer M1 disposed under the drivingtransistor T1.

For example, in FIG. 9, the overlapping layer M1 is disposed under thedriving transistor T1, and further includes a portion M1-1 that extendsunder the third transistor T3. The overlapping layer M1 is connecteddirectly to the second gate electrode G3-2 of the third transistor T3through an opening exposing the extended overlapping layer M1. Since theoverlapping layer M1 receives the driving voltage ELVDD through thedriving voltage application part C-1, the driving voltage ELVDD is alsoapplied to the second gate electrode G3-2 of the third transistor T3.

Compared to the exemplary embodiment illustrated in FIG. 8, in theexemplary embodiment illustrated in FIG. 9, the structure on theinterlayer insulating layer 160 is simplified, and the structure underthe polycrystalline semiconductor layer (e.g., under the gate electrodeG1 of the driving transistor T1) is more complex. The transistor usingthe polycrystalline semiconductor layer generally uses the top gate typeand generally has a more complex structure while disposing the organiclight emitting diode OLED made of the pixel electrode, the organicemission layer, and the common electrode on the transistor.

Accordingly, in the exemplary embodiment illustrated in FIG. 9, thestructure under the polycrystalline semiconductor layer (e.g., under thegate electrode G1 of the driving transistor T1) that is relativelysimple becomes more complex, and the structure of the upper region (onthe interlayer insulating layer 160) is simplified.

Since the area of the pixel PX becomes small at high resolution, thesimple structure on the interlayer insulating layer 160 like in FIG. 9may be advantageous in certain scenarios.

Next, an exemplary embodiment combining the exemplary embodiments ofFIGS. 5 to 8 is described with reference to FIGS. 10 and 11.

FIG. 10 is an equivalent circuit diagram of one pixel of an organiclight emitting diode display according to an exemplary embodiment. FIG.11 is a cross-sectional view of a plurality of transistors included inone pixel of an organic light emitting diode display according to anexemplary embodiment (e.g., the exemplary embodiment of FIG. 10).

Referring to FIGS. 10 and 11, for convenience of explanation, a furtherdescription of elements previously described may be omitted.

The exemplary embodiment of FIGS. 10 and 11 further includes the secondgate electrode G2-2 that applies the driving voltage to the secondtransistor T2 (like the exemplary embodiment of FIGS. 5 and 6), andfurther includes the second gate electrode G3-2 that applies the drivingvoltage to the third transistor T3 (like the exemplary embodiment ofFIGS. 7 and 8).

Compared to the circuit diagram of FIG. 1, in the circuit diagram ofFIG. 10, the second transistor T2 further incudes the second gateelectrode G2-2, and the third transistor T3 further includes the secondgate electrode G3-2. As a result, the second gate electrode G2-2 of thesecond transistor T2 is connected to the driving voltage line 172, andthe second gate electrode G3-2 of the third transistor T3 is alsoconnected to the driving voltage line 172. This structure shifts thechannel characteristic of the second transistor T2 and the channelcharacteristic of the third transistor T3.

In the cross-sectional view of FIG. 11, the characteristics of FIGS. 6and 8 are shown together. For example, compared to the cross-sectionalview of FIG. 3, the second transistor T2 further includes the secondgate electrode G2-2, and the third transistor T3 further includes thesecond gate electrode G3-2. Also, the driving voltage application partC-2 that applies the driving voltage ELVDD to the second gate electrodeG2-2 of the second transistor T2 and the driving voltage applicationpart C-3 that applies the driving voltage ELVDD to the second gateelectrode G3-2 of the third transistor T3 are further included.

The exemplary embodiment of FIG. 11 has the structure in which twodriving voltage application parts C-2 and C-3 are directly connected.According to the exemplary embodiment, two driving voltage applicationparts C-2 and C-3 may only be electrically connected through aconnection part.

As described above, the second and third transistors T2 and T3 includethe top gate. However, according to an exemplary embodiment, the secondand third transistors T2 and T3 may further be formed with the bottomgate and the second gate electrodes G2-2 and G3-2 may be formed with thetop gate. In this case, the gate electrodes G2 and G3 may be formedunder the polycrystalline semiconductor layer to be connected to thescan line 151, and the second gate electrodes G2-2 and G3-2 may beformed on the polycrystalline semiconductor layer so as to apply thedriving voltage ELVDD.

Next, an exemplary embodiment further including a second gate electrodeG4-2 that applies the driving voltage to the fourth transistor T4 isdescribed with reference to FIGS. 12 and 13.

FIG. 12 is an equivalent circuit diagram of one pixel of an organiclight emitting diode display according to an exemplary embodiment. FIG.13 is a cross-sectional view of a plurality of transistors included inone pixel of an organic light emitting diode display according to anexemplary embodiment (e.g., the exemplary embodiment illustrated in FIG.12).

Referring to FIGS. 12 and 13, for convenience of explanation, a furtherdescription of elements previously described may be omitted.

Compared to the circuit diagram of FIG. 1, in the circuit diagram ofFIG. 12, the fourth transistor T4 further includes the second gateelectrode G4-2, and the second gate electrode G4-2 is connected to thedriving voltage line 172. Thus, the second gate electrode G4-2 receivesthe driving voltage ELVDD, and as a result, the channel characteristicof the fourth transistor T4 is shifted.

Compared to the cross-sectional view of FIG. 3, in the cross-sectionalview of FIG. 13, the fourth transistor T4 further includes the secondgate electrode G4-2, and the driving voltage application part C-4 thatapplies the driving voltage ELVDD to the second gate electrode G4-2 isfurther included.

The cross-section of the fourth transistor T4 is now described indetail.

The buffer layer 112 is disposed on the upper barrier layer 111-1, andthe second gate electrode G4-2 is disposed on the buffer layer 112. Thesecond gate electrode G4-2 is covered by the first gate insulating layer141.

The polycrystalline semiconductor layer is disposed on the first gateinsulating layer 141. The polycrystalline semiconductor layer includesthe first electrode S4, the second electrode D4, and the channel layerdisposed therebetween.

The polycrystalline semiconductor layer is covered by the second gateinsulating layer 142, and the gate electrode G4 is formed on the secondgate insulating layer 142. The gate electrode G4 is covered by theinterlayer insulating layer 160.

The driving voltage application part C-4 is formed on the interlayerinsulating layer 160. The driving voltage application part C-4 isconnected to the second gate electrode G4-2 of the second transistor T4though an opening exposing the second gate electrode G4-2. The drivingvoltage application part C-4 applies the driving voltage ELVDD to thesecond gate electrode G4-2. As a result, the channel characteristic ofthe fourth transistor T4 is shifted.

The gate electrode G4 is disposed on the channel of the fourthtransistor T4, and has a width corresponding to (e.g., substantiallyequal to) the width of the channel. The gate electrode G4 may be used asa mask when doping the polycrystalline semiconductor layer.

As described above, the fourth transistor T4 includes the top gate.However, in an exemplary embodiment, the second gate electrode G4-2 thatapplies the driving voltage ELVDD is additionally formed such that thechannel characteristic of the fourth transistor T4 is also shifted.

According to an exemplary embodiment, the fourth transistor T4 mayinclude the bottom gate and the second gate electrode G4-2 may includethe top gate. In this case, the gate electrode G4 may be formed underthe polycrystalline semiconductor layer, and the second gate electrodeG4-2 may be formed on the polycrystalline semiconductor layer so as toapply the driving voltage ELVDD.

An exemplary embodiment including the characteristic (further includingthe second gate electrode G4-2 that applies the driving voltage to thefourth transistor T4) of the exemplary embodiment of FIGS. 12 and 13along with the characteristic of the above-described exemplaryembodiments may be provided. For example, in addition to the exemplaryembodiment of FIGS. 12 and 13, the second transistor T2 may furtherinclude the second gate electrode G2-2 that applies the driving voltage,or the third transistor T3 may further include the second gate electrodeG3-2 that applies the driving voltage. Also, the second gate electrodesG2-2 and G3-2 that receive the driving voltage may be included in boththe second transistor T2 and the third transistor T3.

In an exemplary embodiment, the fifth transistor T5 to the seventhtransistor T7 may also further include the second gate electrode thatreceives the driving voltage.

In an exemplary embodiment described above, only the driving transistorT1 uses the bottom gate structure (the gate electrode is disposed underthe polycrystalline semiconductor layer), and the other transistorsT2-T7 use the top gate structure (the gate electrode is disposed on thepolycrystalline semiconductor layer).

However, according to an exemplary embodiment, the bottom gate structuremay be used in transistors other that the driving transistor T1 suchthat the influence on the characteristic of the transistors other thanthe driving transistor T1 due to the protrusion of the polycrystallinesemiconductor may be reduced.

Next, an exemplary embodiment in which the second transistor T2 and thethird transistor T3 have the bottom gate structure is described withreference to FIG. 14.

FIG. 14 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

In the exemplary embodiment of FIG. 14, unlike in the exemplaryembodiment of FIG. 3, the second transistor T2 and the third transistorT3 use the bottom gate structure. Next, the structure of the secondtransistor T2 and the third transistor T3 is described in detail.

The cross-section of the second transistor T2 is described first.

The buffer layer 112 is disposed on the upper barrier layer 111-1, andthe gate electrodes G2 and 151 are disposed on the buffer layer 112. Thegate electrode G2 (151) is covered by the first gate insulating layer141.

The polycrystalline semiconductor layer is disposed on the first gateinsulating layer 141. The polycrystalline semiconductor layer includesthe first electrode S2, the second electrode D2, and the channel layerdisposed therebetween.

The polycrystalline semiconductor layer is covered by the second gateinsulating layer 142, and the interlayer insulating layer 160 is formedon the second gate insulating layer 142.

The data line 171 is formed on the interlayer insulating layer 160. Thedata line 171 is connected to the first electrode S2 of the secondtransistor T2 through an opening exposing the first electrode S2 of thesecond transistor T2. When the second transistor T2 is turned on, thedata voltage is input to the corresponding pixel PX.

The gate electrode G2 of the second transistor T2 is disposed under thepolycrystalline semiconductor layer such that it has the bottom gatestructure, and the gate electrode G2 has a width corresponding to (e.g.,substantially equal to) the width of the channel. Also, since the gateelectrode G2 is disposed under the polycrystalline semiconductor layer,a mask may be separately utilized when doping the polycrystallinesemiconductor layer.

In the exemplary embodiment of FIG. 14, the second transistor T2 doesnot separately include the structure shifting the channelcharacteristic, and according to an exemplary embodiment, the drivingvoltage ELVDD may be applied and the second gate electrode disposed onthe polycrystalline semiconductor layer may be formed.

Next, the cross-section of the third transistor T3 is described.

The buffer layer 112 is disposed on the upper barrier layer 111-1, andthe gate electrode G3 (151) is disposed on the buffer layer 112. Thegate electrode G3 (151) is covered by the first gate insulating layer141.

The polycrystalline semiconductor layer is disposed on the first gateinsulating layer 141. The polycrystalline semiconductor layer includesthe first electrode S3, the second electrode D3, and the channel layerdisposed therebetween.

The polycrystalline semiconductor layer is covered by the second gateinsulating layer 142, and the interlayer insulating layer 160 is formedon the second gate insulating layer 142.

The connection part 71 is formed on the interlayer insulating layer 160.The connection part 71 electrically connects the second electrode D3 ofthe third transistor T3 and the gate electrode G1 of the drivingtransistor T1 through openings respectively exposing the secondelectrode D3 of the third transistor T3 and the gate electrode G1 of thedriving transistor T1. Since the output of the driving transistor T1 ischanged depending on the voltage of the gate electrode G1, the voltageoutput to the second electrode D3 of the third transistor T3 affects tothe output of the driving transistor T1.

The gate electrode G3 of the third transistor T3 is disposed under thepolycrystalline semiconductor layer such that it has the bottom gatestructure, and the gate electrode G3 has a width corresponding to (e.g.,substantially equal to) the width of the channel. Since the gateelectrode G3 is disposed under the polycrystalline semiconductor layer,a mask may be separately utilized when doping the polycrystallinesemiconductor layer.

In the exemplary embodiment of FIG. 14, the third transistor T3 does notseparately include the structure shifting the channel characteristic,and according to an exemplary embodiment, the driving voltage ELVDD maybe applied and the second gate electrode disposed on the polycrystallinesemiconductor layer may be formed.

Unlike the exemplary embodiment of FIG. 14, only one of the secondtransistor T2 and the third transistor T3 includes the bottom gatestructure. Also, in an exemplary embodiment, one transistor from amongthe fourth to seventh transistors T4 to T7 may have the bottom gatestructure.

In addition to this structure, at least one transistor from among thesecond to seventh transistors T2-T7 may further include the second gateelectrode that receives the driving voltage ELVDD.

In the exemplary embodiment described above, the driving transistor T1further includes the second gate electrode G1-2 on the polycrystallinesemiconductor layer while having the bottom gate structure. However,according to an exemplary embodiment, the second gate electrode G1-2 maybe omitted. This structure is now described with reference to FIG. 15.

FIG. 15 is a cross-sectional view of a plurality of transistors includedin one pixel of an organic light emitting diode display according to anexemplary embodiment.

In the driving transistor T1 according to the exemplary embodiment ofFIG. 15, the second gate electrode G1-2 is omitted, unlike the exemplaryembodiment of FIG. 1.

The cross-section of the driving transistor T1 according to theexemplary embodiment of FIG. 15 is now described.

The overlapping layer M1 is disposed on the upper barrier layer 111-1,and the overlapping layer is covered by the buffer layer 112. The gateelectrode G1 is formed on the buffer layer 112, and the gate electrodeG1 is covered by the first gate insulating layer 141. Thepolycrystalline semiconductor layer is disposed on the first gateinsulating layer 141. The polycrystalline semiconductor layer includesthe first electrode S1, the second electrode D1, and the channel layerdisposed therebetween.

The polycrystalline semiconductor layer is covered by the second gateinsulating layer 142, and the interlayer insulating layer 160 isdisposed on the second gate insulating layer 142.

The driving voltage application part C-1 is formed on the interlayerinsulating layer 160. The driving voltage application part C-1 isconnected to the overlapping layer M1 through an opening exposing theoverlapping layer M1. Also, the driving voltage application part C-1 isconnected to the driving voltage line 172 such that the driving voltageELVDD flows. As a result, the driving voltage ELVDD is also applied tothe overlapping layer M1. The driving voltage application part C-1 maybe a part that extends from the driving voltage line 172 or a part thatis only electrically connected.

The gate electrode G1 is disposed under the channel of the drivingtransistor T1, and the gate electrode G1 has a width corresponding to(e.g., substantially equal to) the width of the channel. FIG. 15 shows astructure in which the gate electrode G1 extends to the right to beconnected to the second electrode D3 of the third transistor T3, and thewidth thereof, except for the extended part, corresponds to (e.g., issubstantially equal to) the width of the channel.

In the driving transistor T1 of FIG. 15, the second gate electrode isnot formed on the polycrystalline semiconductor layer such that a maskmay be separately utilized when doping the polycrystalline semiconductorlayer. According to an exemplary embodiment, the structure of thedriving voltage application part C-1 may be formed of the shape coveringthe channel of the driving transistor T1 such that an exemplaryembodiment that does not use the mask may be implemented.

According to an exemplary embodiment, the overlapping layer M1 may beomitted.

In each exemplary embodiment described herein, the wiring and theelectrode disposed at the same layer may be formed of the same material.The layer disposed on the second gate insulating layer 142 may be formedof the same material, and may be formed by using one mask. Also, eachlayer disposed on the upper barrier layer 111, on the buffer layer 112,on the first gate insulating layer 141, and on the interlayer insulatinglayer 160 may be formed of the same material.

While the present invention has been particularly shown and describedwith reference to the exemplary embodiments thereof, it will beunderstood by those of ordinary skill in the art that various changes inform and detail may be made therein without departing from the spiritand scope of the present invention as defined by the following claims.

What is claimed is:
 1. An organic light emitting diode display,comprising: a substrate; and a pixel disposed on the substrate; whereinthe pixel comprises: an organic light emitting element; and a drivingtransistor that applies a current to the organic light emitting element;wherein the driving transistor comprises: a first gate electrode; and asemiconductor layer comprising a channel, wherein the first gateelectrode of the driving transistor is disposed between thesemiconductor layer of the driving transistor and the substrate, andwherein an overlapping electrode of the driving transistor is disposedbetween the substrate and the first gate electrode of the drivingtransistor.
 2. The organic light emitting diode display of claim 1,further comprising: a scan line; a data line; a driving voltage linetransmitting a driving voltage; and an initialization voltage line,wherein the scan line, the data line, the driving voltage line, and theinitialization voltage line are connected to the pixel.
 3. The organiclight emitting diode display of claim 2, wherein the driving voltage isapplied to the overlapping electrode of the driving transistor.
 4. Theorganic light emitting diode display of claim 2, wherein the pixelfurther comprises: a first switching transistor connected to the scanline and the data line; and a second switching transistor connected tothe scan line and the first gate electrode of the driving transistor. 5.The organic light emitting diode display of claim 4, wherein the drivingtransistor has a bottom gate structure which has the first gateelectrode of the driving transistor disposed under the semiconductorlayer of the driving transistor.
 6. The organic light emitting diodedisplay of claim 5, wherein the first switching transistor and thesecond switching transistor have a top gate structure, and wherein thetop gate structure has a gate electrode disposed on the semiconductorlayer comprising channel.
 7. The organic light emitting diode display ofclaim 6, wherein one of the first switching transistor and the secondtransistor comprises a second overlapping electrode disposed between thesubstrate and the semiconductor layer, and wherein the driving voltageis applied to the second overlapping electrode.
 8. The organic lightemitting diode display of claim 7, wherein both of the first switchingtransistor and the second transistor have the second overlappingelectrodes respectively.
 9. The organic light emitting diode display ofclaim 6, further comprising at least one of compensation transistorsthat compensates an operation of the driving transistor, wherein the atleast one of the compensation transistors has the top gate structure.10. The organic light emitting diode display of claim 9, wherein atleast one of the compensation transistor comprises a third overlappingelectrode disposed between the substrate and the semiconductor layer,and wherein the driving voltage is applied to the third overlappingelectrode.
 11. The organic light emitting diode display of claim 5,wherein the first switching transistor and the second switchingtransistor have the bottom gate structures respectively.
 12. The organiclight emitting diode display of claim 11, further comprising at leastone of compensation transistors that compensates an operation of thedriving transistor, wherein the at least one of the compensationtransistors has a top gate structure, and wherein the top gate structurehas a gate electrode disposed on the semiconductor layer comprisingchannel.
 13. The organic light emitting diode display of claim 1,wherein the semiconductor layer comprises a polycrystallinesemiconductor.
 14. An organic light emitting diode display, comprising:a substrate; a pixel disposed on the substrate; wherein the pixelcomprises: an organic light emitting element; a driving transistor thatapplies a current to the organic light emitting element; and wherein thedriving transistor comprises: a first gate electrode; a semiconductorlayer comprising a channel; and a second gate electrode disposed on thesemiconductor layer of the driving transistor; wherein the first gateelectrode of the driving transistor is disposed between thesemiconductor layer of the driving transistor and the substrate, andwherein an overlapping electrode is disposed between the substrate andthe first gate electrode of the driving transistor.
 15. The organiclight emitting diode display of claim 14, further comprising: a scanline; a data line; a driving voltage line transmitting an drivingvoltage; and an initialization voltage line, wherein the scan line, thedata line, the driving voltage line, and the initialization voltage lineare connected to the pixel,
 16. The organic light emitting diode displayof claim 15, wherein the driving voltage is applied to the overlappingelectrode of the driving transistor.
 17. The organic light emittingdiode display of claim 15, wherein the pixel further comprises: a firstswitching transistor connected to the scan line and the data line; and asecond switching transistor connected to the scan line and the firstgate electrode of the driving transistor.
 18. The organic light emittingdiode display of claim 17, wherein the driving transistor has a bottomgate structure which has the first gate electrode of the drivingtransistor disposed under the semiconductor layer of the drivingtransistor.
 19. The organic light emitting diode display of claim 18,wherein the first switching transistor and the second switchingtransistor have a top gate structure, and wherein the top gate structurehas a gate electrode disposed on the semiconductor layer comprisingchannel.
 20. The organic light emitting diode display of claim 19,wherein one of the first switching transistor and the second transistorcomprises a second overlapping electrode disposed between the substrateand the semiconductor layer, and wherein the driving voltage is appliedto the second overlapping electrode.
 21. The organic light emittingdiode display of claim 20, wherein both of the first switchingtransistor and the second transistor have the second overlappingelectrodes respectively.
 22. The organic light emitting diode display ofclaim 19, further comprising at least one of compensation transistorsthat compensates an operation of the driving transistor, Wherein the atleast one of the compensation transistors has the top gate structure.23. The organic light emitting diode display of claim 22, wherein atleast one of the compensation transistor comprises a third overlappingelectrode disposed between the substrate and the semiconductor layer,and wherein the driving voltage is applied to the third overlappingelectrode.
 24. The organic light emitting diode display of claim 18,wherein the first switching transistor and the second switchingtransistor have the bottom gate structures respectively.
 25. The organiclight emitting diode display of claim 24, further comprising at leastone of compensation transistors that compensates an operation of thedriving transistor, wherein the at least one of the compensationtransistors has a top gate structure, and wherein the top gate structurehas a gate electrode disposed on the semiconductor layer comprisingchannel.
 26. The organic light emitting diode display of claim 14,wherein the semiconductor layer comprises a polycrystallinesemiconductor.